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            <h2 class="title" style="clear: both"><a id="saveret"></a>Saving and Retrieving Data</h2>
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      <p>
                  All data stored using the DPL has one primary index and
                  zero or more secondary indices associated with it.
                  (Sometimes these are referred to as the primary and
                  secondary <span class="emphasis"><em>keys</em></span>.) So to store data under the DPL, you must:
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            <p>
                                  Declare a class to be an entity class.
                          </p>
          </li>
          <li>
            <p>
                                  Identify the features on the class which
                                  represent indexed material.
                          </p>
          </li>
          <li>
            <p>
                                  Retrieve the store's primary index for a
                                  given class using the
                                  <code class="methodname">EntityStore.getPrimaryIndex()</code>
                                  method.
                          </p>
          </li>
          <li>
            <p>
                                  Put class objects to the store using the
                                  <code class="methodname">PrimaryIndex.put()</code>
                                  method.
                          </p>
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      <p>
                  In order to retrieve an object from the store, you use
                  the index that is most convenient for your purpose. This
                  may be the primary index, or it may be some other
                  secondary index that you declared on your entity class.
          </p>
      <p>
                  You obtain a primary index in the same was as when you
                  put the object to the store: using
                  <code class="methodname">EntityStore.getPrimaryIndex()</code>.
                  You can get a secondary index for the store using the
                  <code class="methodname">EntityStore.getSecondaryIndex()</code>
                  method. Note that
                  <code class="methodname">getSecondaryIndex()</code> requires you
                  to provide a <code class="classname">PrimaryIndex</code> class
                  instance when you call it, so a class's primary index is
                  always required when retrieving objects from an entity
                  store.
          </p>
      <p>
                  Usually all of the activity surrounding saving and
                  retrieving data is organized within a class or classes
                  specialized to that purpose. We describe the construction
                  of these data accessor classes in <a class="xref" href="simpleda.html" title="SimpleDA.class">SimpleDA.class</a>. But before you perform
                  any entity store activity, you need to understand
                  indexes. We therefore describe them in the next chapter.
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